Bumped LevelGraph dependency to 0.6.0.

Author: mcollina

package.json

@@ -27,7 +27,7 @@
   "license": "MIT",
   "devDependencies": {
     "mocha": "~1.10.0",
-    "levelgraph": "~0.5.2",
+    "levelgraph": "~0.6.0",
     "level-test": "~1.2.1",
     "chai": "~1.7.2"
   },